Current methods of semiconductor manufacture comprise a process known as "LOCOS," or local oxidation of silicon. Various LOCOS and LOCOS-related processes are described in the following US Patents, each of which is assigned to Micron Technology, Inc. and incorporated herein by reference: U.S. Pat. No. 5,212,111 to Doan, et al; U.S. Pat. No. 5,266,523 to Manning; U.S. Pat. No. 5,358,892 to Rolfson; U.S. Pat. No. 5,393,694 to Mathews; and U.S. Pat. No. 5,405,788 to Manning, et at. With a typical LOCOS process, a thin "pad oxide" layer is formed over a typically silicon substrate, and a patterned nitride layer such as silicon nitride is formed over the pad oxide. The surface of the substrate is then exposed to oxidizing agents. As the formation of oxide under the nitride is inhibited, the substrate oxidizes to form a thicker oxide only in those areas of the substrate uncovered by the nitride. Some encroachment or "bird's beak" under the nitride can also occur which becomes more of a problem as line widths decrease. As line widths decrease, encroachment decreases the width of a channel under a transistor gate to a greater percentage than with a larger device having wider line widths.
Various steps have been taken to decrease encroachment. For example, because thicker pad oxide under the nitride encourages encroachment the pad oxide thickness is decreased. The pad oxide must be thick enough, however, to protect the substrate during formation of the patterned nitride. If the pad oxide is too thin, it can be completely removed in some areas of the substrate when a blanket nitride layer is etched to form the patterned layer and then the silicon substrate will begin to be etched. Etching the silicon is to be avoided as later oxidation of previously etched and pitted substrate forms defects in the silicon which degrades integrated circuit (IC) electrical characteristics.
A process which allows for a thin pad oxide to decrease encroachment and is less susceptible to damaging the substrate from an excessively thin pad oxide would be desirable.